1. Field of the Invention
The field of the present invention relates to laser beam control systems which include a bidirectional beam director to steer incoming and outgoing laser light in the desired direction.
2. Background
Many systems that use lasers to engage or image a remote target also require mechanisms for steering and directing the laser light over a wide operational field of view. In a typical such system, it is desirable to project a high power beam, typically 30-50 cm in diameter or more, over a wide field of view such as a 90-120 degree cone. Some systems may further require the ability to track the outgoing laser light using backscatter or laser light return from the transmitting medium or the target. Receiving the backscatter or laser light return in turn may require that the incoming laser light be steered onto detectors. This ability to precisely steer and point incoming or outgoing laser light is crucial to the operation of many laser weapons, laser illuminators, and laser imaging systems.
Currently many high energy laser (HEL) systems use turrets as beam directors. A turret is usually large, heavy and disruptive to the aerodynamic properties of the platform it is mounted on. Turrets typically require a volume approximately three times the beam diameter on each side. In addition, a turret is usually mounted exterior to the carrying platform (e.g., an aircraft), and, due to its wide volume of swing, it can substantially impair aerodynamic performance which effectively eliminates the option of supersonic HEL operation. One example is the Airborne Laser (ABL) laser turret mounted on the nose of a Boeing 747-400F aircraft. This turret is a large structure with a complex design. It measures 1.5 meters in diameter and weighs 12 to 15 thousand pounds.
Some HEL laser systems use a monolithic single source laser beam while others use a composite beam which is a combination of multiple beams. Using a composite beam offers some advantages: First, it may be advantageous (in terms of power, weight, size and/or cost) to create the laser power in multiple pathways. Second, it may be more practical to correct the phase and tilt of independent beams than correcting a single large beam with a deformable mirror, especially when laser power is projected through an aberrating medium. Third, when installing a high power laser system on an air vehicle, using multiple beams can allow for a conformal installation which does not disrupt the aerodynamic capabilities of the aircraft.
With multiple beams, the conventional beam steering approach is to create a beam director for each beam. Two examples can be given: The first example is the beam director using electrical switching of polarization combined with birefringent gratings, as disclosed in U.S. Pat. No. 6,765,644 to Anderson et al. Current designs of this beam director have about 40 optical components dedicated to each of the beam director modules. Moreover, the components are high cost due to the requirement for excellent wavefront quality and high transmission. The second example is the beam pointing module disclosed in U.S. Pat. No. 7,898,712 to Adams et al., which uses two rotatable Risley prisms. This Risley design includes inside out torque motors and a substantial number of precision components tightly packed together, leading to high cost per steering module.
In addition to high cost, using individual beam directors for each beam of an optical phased array or multi-beam laser system can create three performance problems. First, the mounts and mechanisms around each beam steering module are larger than the exit beam diameter resulting in substantial gaps between the beams. Gaps between the beams act like diffraction gratings and decrease the amount of power in the phased hit spot proportional to the area fill factor. Since the area fill factor of these two devices varies from 25-64% depending on design parameters, a substantial loss of laser capability can result.
The second problem is the slew induced phase ramp which occurs when engaging a rapidly moving vehicle. A Mach 1 target at 3 km range can be slewing across the field of view at 6 degrees per second. This angular slew rate changes the optical path to each of the fixed beam paths equal to the dot product of the vector rate times their vector separation. Thus for a 50 cm diameter laser array, one side will have its optical path to the target changing at 5 cm/sec relative to the other side. In an optical phased array transmitter, this slew in optical path length must be compensated with an opposite slew in the phase of individual beams in order to keep the beams correctly phased on the target. At 1.03 microns wavelength, 5 cm/sec corresponds to a phasing slew rate of 48,500 cycles per second—all of which must be applied to better than 0.1 waves accuracy at each instant across the full array. This poses a significant technical challenge.
The third problem occurs in a high energy laser system where the input beams are each typically diffracting out of a fiber input. Approximately the central 90% of the beam power is typically directed out each local beam director, and the remaining 10% tends to get caught inside the local beam director, causing heating and scatter which degrade operation.
These multi-beam laser system beam directing and steering challenges can be overcome by using a single bidirectional beam director for all beams.
A multi-beam laser beam control architecture that can be configured to use a single bidirectional beam director is disclosed in U.S. patent publication No. 2011-0176565, based on an application filed Jan. 18, 2010, the disclosure of which is incorporated herein by reference in its entirety.
A multi-beam laser beam control and imaging architecture that can be configured to use a single bidirectional beam director is disclosed in U.S. patent application Ser. No. 13/476,380, filed May 21, 2012, the disclosure of which is incorporated herein by reference in its entirety.
Finally, a laser beam control system for a single monolithic laser beam that can be configured to use a bidirectional beam director is disclosed in U.S. patent publication No. 2011/0103410, based on an application filed Mar. 27, 2009, the disclosure of which is incorporated herein by reference in its entirety. In this particular case, a bidirectional beam director can be used to aim the outgoing laser beam at a distant target and at the same time aim the laser light return (caused by the outgoing beam or the target illuminator beam) towards the wavefront sensors of the beam control architecture.